49 Some Malaysian Phytogeographical Problems

49 SOME MALAYSIAN PHYTOGEOGRAPHICAL PROBLEMS. B y E. D . MERRILL, Professor of Botany , Harvard University. Perhaps no part of the l\·orld is more intriguing from the standpoint of phytogeography than is the great equatorial archi pelago lying bet\Yeen southern Asia and Australia. :Malaysia is by far the largest island gronp in the world, lies ,,·holly \Yithin the humid tropics, has great diversity of altitudes np to nearly five thousand metres, and enjoys uniformly high low altitude temperatures, and, except in liibited regions, an abundant rainfall. Almost continuous precipitation occurs over large sections, accompanied by relatively high humidity; other large areas are characterized by alternating wet and dry seasons. These factors, combined with the characters of the soils, the topography, and the position of mountain masses in relation to prevaili ng winds provide optimum conditions for plant grm,·th, and the net result is a flora of tremendous richness and exuberance. The differentia tion of species has perhaps been in part favot·ed by the geoloo·ic development of the region, and its more or less insular character over long periods of time. Under present conditions contiguous parts of the same island may present rather strikingly different floras, \Yhile certain islands separated from each other by onl~· relatively narrow anns of the sea may have very strikingly different vegetative and floristic aspects. Some years ago on the basis of a study of endemism of thos~ parts of Malaysia blessed with published floras or compiled enumerations, such as the Malay Peninsula, Java, Borneo, and the Philippines, I concluded that when the flora was approxi mately known, that in the Pteridophytes and the Spermatophytes combined its total "·ould approximate to 45,ooo species. Thus for the Malay Peninsula, Borneo, and the Philippines, considering all kno\\·n species, it was {ound that at least fifty per cent. of those known from each major unit \Yere endemic. Considering how relatively little \\·as then, and still is, knmn1 regarding the floras of the large islands, including Snmatra, Borneo, Celebes, and New Guinea, I am inclined to consider that this estimate o£ 45,000 species is perhaps too conservative. With a land area in excess of one million square miles. this region (including the Malay Peninsula, which is almost an island, and \Thich is essentially a part of the Archipelago) extends from northern Sumatra to Java, Ne\v Guinea, and the Philippines, .-t region of magnificent distances, extending from north-east to south-east, a distance of over forty-five hundred miles. Among its many thousand islands are found the largest ones in the world : New Guinea, Sumatra, and Borneo, and such large Vol. IX. (1935). so secondary ones as Java, Luzon, 1\lindanao, Celebes, Gilolo, and Timor. The magnitude of some of these major islands is well suggested by Wallace's statement that 'Yithin Borneo "the ,,·hole of the British Isles might be set don·n, and n·ould be surrounded by a sea of forests." This is one of those parts of the world regarding \\"hich ''"e have only an approximate knowledge of its flora. Certain areas, such as parts of the Malay Peninsula, Java, and parts of the Philippines, may be considered to be n·ell-known botanically, ·but in fact only certain very limited parts are thoroughly kno,rn. Vast regions have been only superficially explored, and great areas have never been visited by any collector or botanist. A distinctly high percentage of the known species is known only from the single collections on which the original descriptions are based. We cannot, therefore, make any reasonably accurate com parison between any particular part of this vast region and comparative areas in what may consider to be thoroughly explored parts of Europe and North America. In any consideration of the numerous phytogeographical problems of Malaysia we are handi capped by our ovvn limited knowledge of its exuberant flora, and must realize that future intensive exploration will yield data which may conceivably alter some of our at present accepted axwms. Although the geologic history of Malaysia is reasonably ''"ell understood in its salient outlines much detailed knowledge is lacking. The magnificent work of the Dutch geologists and hydrographers on the geological history of Malaysia forms a basis that must be considered by those biologists interested in the present-day distribution of plants and animals, for the key to present-day distribution to a large degree is found in geolog-ic history. Whatever the causes may have been, it seems to be evident that the great islands of Sumatra, Java, Borneo, and the Palawan-Calamian group in the Philippines were in the Pleiocene Pleistocene at times connected with the Asiatic continent, and Nen· Guinea had the same relationship with Australia. There is much evidence to support the idea that most of the Philippine group, Celebes, Lesser Sunda Islands, and the Moluccas, at this time in geologic history, as "\\ell as in the Tertiary, n·ere con stantly archipelagic, with no direct land connections between eastern and western Malaysia, and hence no direct land connec tions between Asia and Australia since the early Tertiary. Botanists, probably because confronted by such myriads of species, have not been as prone as zoologists to delimit life zones or to map provinces. The attempts of the zoologists to establish lines of demarcation separating the Asiatic and Australian faunas are interesting. Wallace's line extending northward from between Bali and Lombok through the Macassar Strait between Borneo and Celebes is, at least popularly, the best known of these. Its most noteworthy substitute is Weber's Line, farther Gardens Bulletin, S .S. 51 to the east. If these lines are plotted on a hydrographic map the former approximates the eastern boundary of the Asiatic contin ental shelf and the latter the western boundary of the Australian Papuan shelf. It is reasonably good evidence that these two shelves mark the approximate limits of previously existing continental areas. These two more or less stable areas havt been constantly separated by a tri~ngular unstable insular area, its base the Lesser Sunda Islands, its apex Luzon. There is no sharp line of demarcation between the Australian and the Asiatic faunas and floras, but rather this unstable insular area forms a great transition zone that has been in part populated by infiltrations from the south and east and from the north and ,rest. No proposed line is absolute for any single group, much less when all groups are considered. It is a well-known fact that as we progress southward and eastward from Asia, the continental types of animals and plants become fewer and fewer, and as we progress northward and eastward from A."ustralia, the Australian types likewise decrease even more markedly. This is exactly what we would expect tf our present concept of the geologic history of the region is approximately correct. One should not make the mistake of drawing final conclu sions on the basis of the present-day distribution of any one group of organisms, whether it be the molluscs, insects, reptiles, birds, freshwater fishes, mammals, or plants, and much less on any single family or genus within these major groups; and yet there are conspicuous cases of broad generalizations that have been founded on the study of a single family, perhaps even a single genus. While some such generalizations may ultimately provt> to be correct, it is always safer to consider what is known regard ing the geographic distribution of other and perhaps totally unrelated groups and to attempt to correlate such data vvith what is known of the geologic history of the entire region. It is a safe assumption that the different groups have been developed and have attained their present-day distribution at different periods in geologic history; and that while some groups were rapidly differentiating, others were perhaps static or were retrograding. That the Philippine flora, as to the Myrtacec:e, shows an un· mistakably close relationship with that of north-eastern Australia is no reason for considering that the entire Philippine flora and fauna is allied to that of Australia. If one studies only the Dipterocarpacere of the Philippines one would conclude that its flora is overwhelmingly allied to that of Borneo, Sumatra, and the Malay Peninsula. Clearly here, as with other groups, the periods of development and expansion have occurred at different times in geologic history, periods that permitted the distribution 0£ one group perhaps restricting that of another. Vol. IX. (1935). 52 Botani_cal collectors who have explored Malaysia compared \Yith those who have \Yorked in Europe and in Xorth America, north of Mexico, have been fe,,:, and the number of botanists who have familiarized themselves " ·ith \\·hat has been published is fewer still. For all practical purposes those botanists \\·ho have concentrated on a study of the floras of the :\Ialay Peninsula, Sumatra, Java, Borneo, the Philippines, and ~e\\" Guinea, in the past t\YO hundred years are merely the pioneers, and their \York should be considered as that of pioneers, to be correlated, polished, and evaluated by future generations of botanists who \Vill have infinitely more to work with than did those ,,·ho first exploited the fertile fields of Malaysian botany. It "·ill be many years before the essential data are assembled on \Yhich \Ye may base final conclusions. Even such work as has been done by various taxonomists on the Malaysian flora \\'Orking in scattered centres in Europe, England, and America, in Manila, Singapore, Buiten zorg, and Calcutta, has not been \\"ell correlated. :M:uch needs to be done to prove the validity or non-validity of some thousands of proposed species.

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